Mold sulfonation system

Abstract

An in-mold sulfonation system for sulfonating the surface of plastic articles such as plastic containers during molding. Sulfur trioxide is introduced into a pre-mold chamber, converted to a relatively high pressure gaseous sulfur trioxide, and then injected into a mold such as a blow mold immediately before or immediately following full expansion of the formed plastic article in the mold cavity. Neutralization with a gaseous neutralizing agent follows. By controlling the timing and temperature of the in-mold sulfonation step it is possible to produce a plastic article having an effective barrier layer but containing a minimum of undesirable rinseable salts.

What is claimed is: 
     
       1. A system for simultaneously molding and sulfonating plastic articles, comprising (a) a mold having a cavity in which a thermoplastic resin is molded to produce a plastic article,   (b) a pre-mold chamber in fluid communication with said mold,   (c) a source of liquid sulfur trioxide in fluid communication with said pre-mold chamber,   (d) means for injecting a charge of liquid sulfur trioxide from said source of liquid trioxide into said pre-mold chamber,   (e) means for converting said charge of liquid sulfur trioxide to a relatively high pressure gaseous sulfur trioxide,   (f) means for applying to said thermoplastic resin in said mold heat and pressure at a forming temperature and pressure to form said thermoplastic resin into a plastic article and means to cool said plastic article after forming, and   (g) means for introducing said relatively high pressure gaseous sulfur trioxide into said mold cavity before any significant cooling of said plastic article takes place whereby said gaseous sulfur trioxide contacts a surface of the formed plastic article in said mold and sulfonates said surface.   
     
     
       2. The system of claim 1 further including means for introducing a gaseous neutralizing agent into said mold. 
     
     
       3. The system of claim 1 wherein said mold is a blow mold. 
     
     
       4. The system of claim 3 wherein said means for injecting sulfur trioxide into said pre-mold chamber is a pump for injecting liquid sulfur trioxide. 
     
     
       5. The system of claim 4 wherein said means for introducing said gaseous sulfur trioxide into said mold cavity is a value in the fluid communication line between said mold cavity and said pre-mold chamber. 
     
     
       6. The system of claim 5 wherein said valve and said pump cannot open or operate at the same time. 
     
     
       7. The system of claim 1 wherein said mold is a counter-pressure injection mold.